1. Field of the Invention
This invention relates to the field of liquid dispensing measurement and control. More specifically, the invention comprises a system and method for accurately correlating liquid level measurements within a storage tank to the volume or mass of liquid contained within the tank. Among other things, the inventive system may be used to confirm the accuracy of a dispenser flow meter.
2. Description of the Related Art
Many different types of liquids are stored and later dispensed. Most dispensed liquids are purchased on the basis of a price per unit volume. One common example is the dispensing of liquid fuels such as gasoline or diesel fuel. Gasoline will be used as an example in the following descriptions of the prior art, but the reader should bear in mind that the prior art encompasses many different liquids and the inventive concepts described in this disclosure likewise encompass many different liquids.
Gasoline is stored in tanks. These are often underground tanks but many tanks are above ground. In fact, the recent trend is toward above-ground tanks. The fuel dispensed from any type of storage tank is generally passed through some type of flow meter to accurately determine the volume of fuel actually transferred to the tank of a vehicle. In addition, it is important for the fuel vender to accurately monitor the level of liquid within the underground storage tank. This permits the vendor to monitor the inventory and forecast the need for replenishment.
The amount of fuel within an underground tank is most often monitored using a linear depth meter. The depth meter can be quite accurate. However, the correlation of measured depth to tank volume is often not very accurate. The tanks are accurately measured when they are newly fabricated and free-standing. A depth-to-volume correlation chart is then created for each individual tank. This process is traditionally called “strapping the tank.” “Strapping” refers to the process of using marked steel straps to precisely measure the circumference of the tank at fixed intervals along its length (fir a horizontally-oriented tank) or along its height (for a vertically-oriented tank). The circumference measurements are used to build a depth-versus-volume chart for a particular tank. This correlation chart is often referred to as a “strapping chart.”
Strapping charts may also be made by adding precisely-measured liquid volumes to a tank and recording the resulting depth on a linear tank gauge. This approach accommodates complex tank geometry (such as a ribbed wall) and manufacturing tolerances. It can produce an accurate strapping chart—at least for the geometry that exists at the time of manufacturing and before the tank is installed.
One may in theory develop an equation for converting linear depth measurements into corresponding volumes, but this is quite complex for even a basic tank design. A simple example serves to illustrate this point. Consider a cylindrical tank with the central axis of the tank lying horizontally. The tank has a radius r and a length l. The ends of the tank are assumed to be perfectly flat plates (unrealistic but this assumption greatly simplifies the calculation). The wall of the tank is assumed to be a perfect cylinder (also unrealistic). A coordinate system is created with the origin lying on the cylinder's central axis. The vertical position of the surface of the fuel within the tank is denoted as y. Thus, y varies from −r to +r. The equation for the volume v may then be written as:v=r2l√{square root over (1−y2)}y+cos−1(−y)
The same equation may be derived from geometry or by using calculus. Either way, the reader will note that the relationship between the depth y and the volume v is rather complex, despite the simplifying assumptions made in this example. If one assumes semi-hemispherical or hemispherical end caps, the relationship becomes much more complex. Further, the assumption of a cylindrical wall is unrealistic. Tanks are theoretically cylindrical but the manufacturing tolerances are significant. For these reasons an equation relating the depth measurement to the volume is not commonly used in industry. Instead, the empirically-derived “strapping chart” is used. Even this approach has problems, however.
When a tank is installed its geometry is invariable altered. An above-ground tank distorts as fuel weight is added. It is often also distorted by differential heating (The side of the tank facing the sun may be much hotter than the other side). A buried tank may see even greater distortion. When a tank is buried in the ground, the soil under it settles and the addition of the substantial fuel weight causes the tank to deform in unpredictable ways. An error in the strapping chart is thereby introduced. If left uncorrected, this error will persist for the entire useful life of the tank.
In addition, the amount of tank deformation is often not static. Soil subsidence may increase over time. The moisture content of the soil will also change over time. The result is that one cannot say with certainty that soil subsidence will increase to a point and then stabilize. In some installations the tank deformation may even be cyclic.
FIG. 1 provides a simplified elevation view of a buried gasoline storage tank connected to a dispenser. Tank 10 is buried beneath concrete slab 18. The tank is typically surrounded by a stabilizing media 20—often a mixture of gravel and smaller particles. Fuel 12 occupies the bottom of the tank up to surface 38. Ullage 40 lies above surface 38 (“ullage” being an old English term for the amount by which a container falls short of being full).
Several access ports are typically provided through slab 18 to tank 10. Fill access 14 provides external access to fill pipe 16. The fill pipe is used to transfer fuel from a tanker truck into the underground tank. In this example it is simply a vertical pipe with an access cover at its upper end. The bottom end is open so that fuel pumped into the top of the pipe flows down and into the tank.
The access near the middle of the tank provides an entry point for an automatic tank gauge (ATG probe 24). This device is used to determine the current linear position of surface 38. Probe head 26 is mounted to the top of ATG probe 24. Automatic tank gauge controller 22 communicates with probe head 26 (This may be a wired or wireless connection). The automatic tank gauge controller is usually remotely located—such as in a nearby building. The controller monitors and records the liquid level. More sophisticated examples include an associated memory device that stores the strapping chart for the specific tank and a processor that converts the measured liquid level into a “gallons remaining” readout. Very sophisticated examples may even monitor the volume of fuel dispensed and compare this to the calculated value for the volume remaining in the tank. This feature is sometimes referred to as an “inventory reconciliation.”
FIG. 12 shows an elevation view of a sophisticated automatic tank gauge. This version includes the ability to separately monitor the position of two independent floats—fuel level float 120 and water level float 122. Fuel level float 120 is configured to float on surface 38 of the fuel contained within the tank. Water level float 122 is configured to measure the depth of any water contained in the bottom of the tank. The density of the water level float is configured so that it sinks through the fuel but floats on the water (owing to the fact that the density of hydrocarbon fuels is less than the density of water). Water level float 122 floats on fuel water/interface 126. ATG probe 24 precisely reports on the depth of surface 38 and on the depth of fuel/water interface 126 (assuming that water is found in the bottom of the tank).
In addition, this particular ATG probe 24 includes five separate temperature sensors 124. These provide temperature information to automatic tank gauge controller 22 (see FIG. 1) which may then be used in different ways. The controller “knows” the depth of fuel in the tank. It therefore “knows” which temperature sensors are wetted by fuel and which are not. It may use the temperature readings from the wetted sensors to determine an average temperature for the fuel in the tank.
The access port shown to the left in FIG. 1 provides access for intake line 30. This intake line is connected to pump 28. The pump draws fuel out of tank 10, pressurizes it, and sends it out through discharge line 32. Vent 34 vents any accumulated fuel vapors to the atmosphere or into a collection system. The discharge line leads to dispenser 36.
Dispenser 36 is a dispensing station familiar to those skilled in the art. Modern versions typically include the ability to dispense three or more different fuels (such as “regular,” “mid-grade,” “premium,” and possibly diesel). It includes dispenser nozzles configured to fit into a vehicle's filler neck. It also includes payment-receiving devices such as a credit card reader. Display devices are also included to display selections made, price charged, and volume dispensed.
Multiple discharge lines 32 typically feed into a single dispenser 36. For example, a dispenser that is able to dispense three different types of fuel may have three different discharge lines leading in. Most dispensing stations have multiple dispensers. However, most stations also only have a single underground tank for each type of fuel sold. As an example, a station selling three different grades of gasoline would have three different storage tanks but might have six or more dispensers. In that case discharge line 32 would be split so that is can feed all six available dispensers. The single pump 28 pressurizes the distribution network for a particular fuel type to all the dispensers. Each dispenser contains a metering device for measuring the volume of fuel dispensed but typically does not contain a separate pump. Those skilled in the art will know that “mid-grade” fuels are often created on-site by blending regular and premium grades, but even these facilities would have at least two tanks and two lines leading to each dispenser.
FIG. 7 depicts some of the internal components of dispenser 36. The particular dispenser shown dispenses three different grades of gasoline (low-grade, mid-grade, and premium). Three discharge lines (32L, 32M, and 32H) arrive at the dispenser from three different underground tanks. Each discharge line typically includes a valve 82 regulating flow of the particular fuel into the dispenser. Downstream of each valve 82 is a piston flow meter 62. A piston flow meter is a precise device that rotates a fixed amount for each volume of fuel passing through it. In older designs the piston flow meter would drive a volume and price display through a set of gears. In modern dispensers the rotation of each piston flow meter is monitored by a rotary encoder 80. This device creates a train of electronic pulses that is counted and analyzed by a separate processor. The processor translates the pulses into the volume and price display (now typically a graphical display such as a color LCD).
A nozzle feed line 88 leads out of each piston flow meter and ultimately leads to the dispensing nozzle itself. As those skilled in the art will know, a separate flow control valve exists on the nozzle. Check valves are used to prevent cross-contamination among the different fuels.
FIG. 6 shows a single piston flow meter 62 in greater detail. Those knowledgeable in the field of fuel dispensing will recognize this device as a standard one that has been used for many years. Its use is required by many regulatory agencies and—while it is an old design—its continued use is expected.
Input pipe 68 allows fuel to enter the pump and output pipe 70 carries the same fuel out of the pump. Housing 64 includes four perpendicular cylinders—each of which is covered by a cylinder cover 66. Each cylinder includes a linearly translating metering piston. Flow through the pump causes the metering pistons to move back and forth over their range of travel. The motion of the metering pistons rotates the meter output shaft 78. The result is that each rotation of meter output shaft 78 corresponds to a precisely-known volume of fuel passing through the pump.
Rotary encoder 80 is coupled to meter output shaft 78. As described previously, the rotary encoder typically provides a pulsed output that allows a separate processor to monitor the rotation of the output shaft and thereby infer the volume of fuel dispensed.
Piston flow meters usually have a calibration wheel 72. Rotating the calibration wheel alters the volume of fuel dispensed per revolution of meter output shaft 78. The calibration wheel is often set by a “weights and measures” governmental regulatory agency. Once it is set lock pin 74 is inserted and a tamper-proof calibration seal 76 is put in place. These devices prevent any unknown movement of the calibration wheel.
Of course, all machines containing moving parts wear over time. The cylinder bores grow larger with wear and internal leakage past the pistons may increase with time. For these reasons and others a piston flow meter does not remain calibrated forever. Its output will drift over time. This “meter drift” phenomenon has long been known. It is the reason that piston flow meters must be periodically recalibrated. New measurements are made during the recalibration process and the calibration wheel is moved to a new position and locked in place.
The calibration process is manual and it can be time consuming. A fixed calibration container (often called a “prover”) is filled with fuel while the measured volume from the flow meter is observed. The flow meter is then adjusted to ensure that the volume it measures matches the volume dispensed into the prover. This process may require more than one fill cycle of the prover.
State statutes often impose fines for any miscalibration that “shorts” the consumer. If such a miscalibration is deemed deliberate, the operator can be fined up to 10,000 U.S. dollars per day. There is generally no fine for a miscalibration in favor of the consumer. However, an error in favor of the consumer effectively gives away free fuel and no operator can afford to do that for long. Thus, there is a strong incentive to keep the flow meters in calibration.
Recalibration intervals are somewhat arbitrary. For some piston flow meters the specified interval is too long and for others it is too short. In the prior art it is not possible to accurately measure the drift of a piston flow meter. Thus, it is not possible to directly know whether the meter remains within a specified tolerance. The setting of a somewhat-arbitrary recalibration interval is intended to ensure that the vast majority of piston flow meters in use will remain in calibration. As one would expect, the way to ensure this is to use a conservative recalibration interval. The result is that many meters will be recalibrated when there was no need to recalibrate. Since recalibration is rather expensive, both in terms of labor and down-time, the current system is inefficient.
The volume and temperature of fuel delivered into an underground tank is accurately known. If one could accurately measure the volume and temperature of fuel in the tank over time, one could then infer the volume dispensed and compare this value with the value measured by the piston flow meter. This comparison would then show whether the piston flow meter accuracy had “drifted” outside the allowable tolerance. Unfortunately, determining the volume of fuel contained within a tank is not an easy problem.
As explained previously, this is most often done using a “tank strapping” chart. FIGS. 2 through 5 illustrate some of the problems experienced when applying a “tank strapping” chart to convert a measured linear depth to a volume of fuel remaining. Systems for converting a measured depth to a remaining volume are fairly complex. FIG. 2 illustrates the cross section of a perfect cylinder having perfectly flat end plates. In FIG. 2(A) the tank is nearly empty. In FIG. 2(B) the tank is about 50% full. From even a casual visual inspection the reader will observe that the same change in measured depth (“Δh”) equates to a very different change in volume for the two examples. The depth change in FIG. 2(B) represents a much wider “slice” of the tank.
Many tanks employ semi-hemispherical end caps and a ribbed main section. The geometry involved in these cases is much more complex, even when the tank is first completed and not yet buried in the ground. When the tank is buried and loaded with fuel a more complex shape results. FIG. 3 depicts a cross section through a buried tank. Circular section 42 represents the ideal, undeformed and circular cross section. Slumped section 44 represents the shape that actually results from the installation process. The degree of slump and the shape of the slump is largely unpredictable.
FIG. 4A depicts the fact that most tank installations are not perfectly level. Some slope angle (“α”) will be present. In fact, some slope is often deliberately introduced so that the tank will have a known low point. The slope angle is exaggerated in FIG. 4 for purposes of visualization. However, even a small slope angle means that surface 38 will not be parallel to the tank's central axis. This fact makes the correlation of depth to volume still more complex.
FIG. 4B depicts the fact that most tank installations also contain some amount of roll angle (“β”). The roll angle may cause the automatic tank gauge probe to be non-perpendicular to the surface of the fuel in the tank. An additional error is thereby introduced.
Finally, FIG. 5 depicts the effects of uneven subsidence during installation. The right end of the tank (with respect to the orientation in the view) has subsided more than the left end. The tank no longer has a linear centerline. Instead, the centerline may be more of a spline.
Other distortion phenomena exist in some installations. Even if one can accurately determine an equation or empirical table for correlating depth to volume as a tank is manufactured, the correlation will not tend to remain accurate after installation. Further, the correlation will tend to change over time.
There are now laser-based systems that actually allow a strapping chart to be created in situ by lowering a measurement system into the tank. Even these are prone to error, however, since the full tank will move with respect to the empty tank and the tank geometry will tend to change over time due to soil subsidence, etc.
Despite these issues there is no doubt that an accurate linear depth measurement can be obtained for the tank when it is completely full. If one can precisely measure the quantity of fuel dispensed over time then it seems at least theoretically possible to create an in situ strapping chart by starting with a full tank and measuring the amount dispensed and the linear depth as the tank goes down. There is another significant problem with this approach, however.
A piston flow meter as shown in FIG. 6 measures the volume of fuel dispensed. Assuming an underground tank is not leaking (and not venting), the mass of fuel contained within the tank remains constant. However, because the density of most fuels is temperature-dependent, the volume of fuel (as opposed to the mass) changes over time.
The density of gasoline, for example, is much more temperature dependent than that of water. At 20° C., the rate of change in the density of gasoline with respect to temperature is about 4.5 times that of water. This strong temperature dependence has long been a recognized problem in fuel dispensing and a table of correction factors is used to compensate for it. Wholesale gasoline sales use these correction factors to ensure that the wholesale purchaser pays for the delivered volume corrected to a standard temperature.
Within the fuel dispensing industry, a standard temperature of 15° C. or 60° F. is used (even though these two values are not precise equivalents since 15° C. is actually 59° F.). A table of correction factors is used to correct for non-standard temperature. The term “gross sale” refers to a volume sold at the actual dispensing temperature, uncorrected from any deviation from the defined standard temperature. The term “net sale” refers to a volume corrected for non-standard temperature. Thus, on a hot day a gross sale transfers relatively less mass and on a cold day a gross sale transfers relatively more mass. A net sale, on the other hand, is supposed to transfer the same mass no matter what the ambient temperature is.
When fuel is transferred from a tanker truck into an underground storage tank this is considered a “wholesale” transaction and temperature correction is routinely applied. The overarching concept of temperature correction is to price the transaction at a unit price per unit volume—where the unit volume is the volume that would have been measured if the temperature had been dead on the standard temperature of 15° C. Another way to state this is that the price paid is effectively a price per unit mass transferred and the correction factors are used to convert measured volume into actual mass. The reader should bear in mind, however, that the industry does not generally speak of these transactions as mass transfers.
Some examples serve best to illustrate these concepts: In all cases the transfer is measured as a volume of fuel. If the temperature of the fuel being transferred is 15° C. then no correction is required. In the standard table the “correction factor” for this condition is simply 1.0000. Thus, if 1,000 U.S. gallons are transferred at a wholesale rate of 2.00 U.S. dollars per gallon and a temperature of 15° C. (approximately 60° F.), then the payment will be 2,000 U.S. dollars.
Consider next what happens if the fuel being transferred has a temperature of 25° C. (approximately 77°). According to the American Petroleum Institute (“API”), the volume correction factor at 25° C. is 0.9874 (Note that there is no correction for variations in atmospheric pressure as its effect is negligible). A temperature of 25° C. is 100 above standard and the correction factor should be applied. The volume for the transaction is adjusted as 1,000 U.S. gallons multiplied by 0.9874 for a net result of 987.4 temperature-compensated gallons purchased. The payment will then be 2.00 multiplied by 987.4, or 1,974.80 U.S. dollars.
In the United States, however, retail fuel purchases have not traditionally been subject to temperature compensation. Returning to FIG. 7, the reader will note that the retail dispenser 36 only contains volumetric flow measuring devices (piston flow meters 62). Thus, the temperature of the fuel being dispensed is not normally measured. While the fuel inventory control system might accurately “know” the volume of fuel being dispensed, it is difficult to correlate this amount to tank volume because of the fuel's unknown density variation.
Storage tanks are buried to an average depth of about 2 meters and surrounding soil temperatures at that depth remain fairly constant—though they do increase and decrease with the changing seasons. If gasoline remained in an underground tank stored for a long time it would likely reach a stable temperature that is comparable to the temperature of the surrounding soil. One could then use this as a standard temperature and accurately translate the measured dispensed volume into tank depth measurements.
But, even a medium-sized station will sell 10,000-15,000 gallons of fuel per day and this means that a tanker truck visits the station about once per day (and multiple times for larger stations). Most of the volume within any given tank turns over in 24 hours. Most modern underground storage tanks are made of non-metallic materials such as fiberglass. These tanks tend to be good thermal insulators. Thus, the fuel placed in an underground tank will not change temperature very rapidly. It may take a day or more to reach the temperature of its surroundings.
Fuel loaded into the tanker truck tends to be stored above ground and the tanker truck itself is of course above ground. Further, the tanker truck uses a thermally conductive tank that is exposed to rapidly moving ambient air. The result is that the fuel within a tanker tends to have a temperature that is close to the average ambient air temperature. In Minnesota in January this would be −11° C. In Florida in July this would be 30° C.
The temperature of the surrounding soil tends to be site-specific. In Minnesota in January the soil temperature at a 2 meter depth averages about 7° C. (45° F.). In Florida in July the deep soil temperature averages about 26° C. (78° F.). The fuel delivered from the winter tanker in Minnesota must increase in temperature over a spread of 18° C. The fuel delivered from the summer tanker in Florida must decrease in temperature over a spread of 4° C.
In either case the temperature variation complicates the reconciliation of fuel volume remaining in the tank against the volume of fuel that has been dispensed. In the case of the winter-time Minnesota tanker, if 1,000 gallons of cold gasoline are loaded into an underground tank and allowed to rest until reaching the ambient deep soil temperature, the 1,000 gallons will expand to be 1,042.5 gallons. In other words, one could dispense 42.5 gallons from the tank but the linear depth gauge in the tank would still show the tank to be absolutely full.
In the case of the summer-time Florida tanker, if 1,000 gallons of hot gasoline are loaded into an underground tank and allowed to rest until reaching the ambient temperature, the 1,000 gallons will contract to be 994.8 gallons. Even with no dispensing, 5.2 gallons will seem to “disappear.”
In recent years very accurate liquid densitometers have become available. These devices are able to accurately measure the density of a liquid as it is dispensed. Very accurate tank depth gauges are also available. A new system that takes advantage of the capabilities of these sensing devices is desirable. In particular, a system that is able to accurately reconcile measurements of dispensed fuel against measurement of fuel retained in the tank is desired. The present invention provides just such a system.